Getting confused about 1 way, 2 way & 4 way stretch & buying it

[Katymac]

Help!!

4 way (I thought) was length, width & diagonally
2 way (I thought) was length, width

So is one way stretch just length or width? & does it matter which?

& what would it be advertised as?

2 way stretch on stretches one way usually selvedge to selvedge (if you pull the selvedges it will stretch) 4 way stretches horizontally and vertically.

That kind of makes sense

& 1 way stretch? That can't really exist can it?

Sorry - I forgot to say 'thank you' for replying

Thank you